This invention relates to a circuit for measuring the frequency of a signal, and in particular to a circuit that rejects noise and spurious signals while accepting valid signals for frequency measurements.
In systems that employ circuits to compensate for timing errors present in a processed signal, the presence of noise, spurious input signals, missing signals or other signal condition anomalies adversely affects the performance of the compensation circuit. One undesirable result is that systems relying on such compensation circuits take a relatively long time, over many periods of the input signal, to recover to normal operating conditions following the occurrence of a signal anomaly. Also, in order to determine the frequency of the incoming pulse signal it is common to count the number of pulses or transitions that occur within a predetermined period. In such systems, it takes a relatively long time to determine the frequency, whereupon frequency and phase compensation and thus timing stability are unduly delayed to the detriment of the system operation. Therefore it would be desirable to reject rapidly those signal anomalies such as noise, and to accept rapidly only those signals which are within defined parameters and, therefore, considered valid.
The invention overcomes the disadvantage of previous mention by providing a circuit for rapidly and continuously measuring the frequency of a signal which may contain noise. The circuit incorporates a frequency measuring circuit, and an averaging circuit that continually averages previous frequency measurements over a long term. The difference in frequency between the averaged value and the current frequency measurement provides an error signal. The error signal is compared to a threshold value to determine whether a signal is to be accepted as valid or rejected as noise. The threshold is adjusted for each period defined by two adjacent pulses by incrementing by one count when an existing error is found to be greater than the threshold value. If the existing error is less than the threshold then the threshold value is decremented. Thus the threshold automatically is adjusted for successive periods between adjacent pulses, which enables a rapid and accurate determination for qualifying valid pulses.
The circuit is particularly useful in a time base correction system for a video tape signal processing apparatus when the system senses that the frequency of the tape clock is characterized by a large error.